Microlithography refers generally to any of several processes by which patterns with small features are copied from a master image to an object such as a silicon wafer. One type of microlithography, called photolithography, is often used in semiconductor manufacturing to define a layer of an integrated circuit. In projection photolithography the image of a glass photomask is projected on a silicon wafer that is coated with a photographic emulsion or photoresist. The exposure of photoresist in a mask aligner is analogous to the exposure of photographic film in a camera.
A glass photomask has patterns of thin metal on its surface. These patterns are usually created by electron beam lithography in which a precisely controlled electron beam traces out shapes under computer control. The electron beam illuminates an electron beam resist which has been applied to a glass mask substrate coated with a thin layer of metal. After the resist is developed, the metal layer is etched away to form the desired pattern.
In contact photolithography, instead of projecting the image of a mask onto a wafer (or other substrate), one presses the mask itself onto the wafer. The wafer is then exposed through the mask by illumination from a bright light source. In general, contact photolithography is not capable of reproducing as small features as projection photolithography, but it is considerably less expensive.
One of the main reasons that photolithography is so important in integrated circuit manufacturing is that patterns representing transistors and other circuit elements with very small features may be printed over and over on silicon wafers. State of the art photolithography systems now print with better than 100 nanometer resolution.
Recently, researchers have taken advantage of microlithography techniques to create very small mechanical devices instead of electronic circuits. These devices, sometimes known as “micro-electro-mechanical systems” or MEMS have found applications in devices as diverse as movie projectors and accelerometers. MEMS are often created using surplus microlithography equipment procured from integrated circuit manufacturers. MEMS technology is still relatively new and normally does not require state of the art lithography tools.
Even more recently, researchers in chemistry and biology have realized the value of printing very small patterns for their experiments. For example, biologists have created DNA array chips which enable them to perform thousands or even millions of simple DNA experiments simultaneously. Chemists and biologists have found great value in printing thin layers of chemical and biological materials. The simplest and least expensive method of printing these materials is simply to stamp them using a method very similar to stamping ink patterns on paper with a rubber stamp. The new method is variously called “microcontact printing”, “soft lithography” or simply “precise pattern transfer.”
One of the innovations inherent in precise pattern transfer was the discovery that elastomeric stamps, especially those made from poly-dimethylsiloxane (“PDMS”) are capable of stamping patterns with feature sizes less than one micron. Precise pattern transfer by PDMS stamping has become a very popular research tool in chemical, biological and MEMS research. Typically a PDMS stamp is created by molding liquid PDMS on an etched mold originally defined photolithographically. After the PDMS cures into a rubbery state it may be peeled off the mold, wetted with various “inks” and stamped on flat or even a curved substrates.
Precise pattern transfer brought the power of microlithography to researchers in diverse disciplines. However, precise pattern transfer as currently practiced has critical limitations compared to photolithography. For example, it's hard to line up the stamp with pre-existing patterns on the substrate.
Alignment of the stamp with substrate features is critical to all but the simplest applications of precise pattern transfer. Without alignment capability stamped patterns can only be roughly located on the substrate and aligning subsequent patterns to previously created patterns is difficult. Crude alignment is currently done by hand. It would be highly desirable to have at least a semi-automatic alignment system for precise pattern transfer by stamping.
Another limitation of stamping techniques is that it is nearly impossible to orient the surface of the stamp relative to the substrate surface with precision. When a stamp is applied by hand, or even with the aid of simple mechanical devices, one part of the stamp tends to touch the substrate surface before the rest. In other words, the stamp is tilted with respect to the substrate.
A tilted stamp leads to at least two problems. First, the stamp may be distorted when only one part of it is touching the printing surface while other parts do not. Second, a tilt will lead to uneven pressure over the surface of the stamp when the stamp is in contact with the substrate. Uneven pressure leads to uneven printing and distortion of printed features. It might seem that slight tilts and variations in stamping pressure would not matter, but in fact control of these parameters is critical when stamped feature sizes are in the micron range.
The limitations of precise pattern transfer are widely known and have inspired several researchers to try to improve stamping techniques. Schueller (see US Patent Application Publication 2003/0047535, incorporated herein by reference), for example, recognizes some of the limitations of current stamping techniques and presents a list of conventional methods to solve current problems.
Kendale (see US Patent Application Publication 2003/0213382, incorporated herein by reference), proposes a system in which a spring-supported stamp is brought into contact with a test substrate for purposes of determining the proper orientation of the stamp in subsequent printing operations.
Hougham (see U.S. Pat. No. 6,656,308, incorporated herein by reference) teaches a process for making elastomeric stamps flatter and less prone to distortion when released from their molds. This method improves the quality of stamped patterns and is a step toward making microcontact printing useful in microcircuit applications.
Despite the progress made by these and other researchers in the field there is a strong need for a simple system for precise pattern transfer by stamping. Ideally such a system would both level a stamp with respect to a substrate and allow the stamp to be aligned to features on the substrate prior to coming in contact with the substrate. It would also be desirable for leveling and aligning procedures to be carried out automatically or under an operator's supervision.